In the field of volatile memories, devices which are used to store binary data, many types of memory cells have been widely used. Two frequently used memory cells include the DRAM and SRAM cell. The former is a dynamic random access memory which is a storage device that needs to be refreshed periodically in order to preserve the data stored therein. The latter is a static random access memory which is a storage device which retains its state so long as power is applied. It does not need to be refreshed as a DRAM does.
The typical DRAM cell consists of a single transistor and a capacitor with the state of the charge on the capacitor representing the data stored in the cell. In the typical SRAM, the cell consists of 6 transistors configured to form a latch. The state of the latch represents the data stored in the cell.
The write circuitry for a typical DRAM consists of a pair of signals acting on the transistor, one being known as a word line (row) signal and the other being a bit line signal. In the SRAM cell, there is a single word line but there are two complementary bit lines (column) required to write data into the cell. This is simply due to the fact that the SRAM cell may have to be forced to change state by the signals applied thereto. The cell thereafter will remain at the state to which the cell had been forced. Because of this requirement to force a state change in a multi-transistor latch, the time and power required to produce a state change in a SRAM cell is greater than for a typical DRAM.
As is well known, memory cells are sensitive to certain forms of radiation which may cause a state change. This problem has been addressed in many ways including using error detection and correction schemes. In the case of SRAM devices, another approach to making the memory cell less sensitive to radiation has been to design the cell in a manner that requires the application of higher power to the cell in order to force it to change state. Logically, this approach should work because the typical radiation encountered in military, aircraft and satellite applications is high enough so that cell design changes can readily make the cell much more difficult to change state. As a result, the cell becomes harder to change from one state to another and typically takes longer than for SRAM cells which are more radiation sensitive.